202 SUB-CLASS (AND ORDER) TELEOSTEI. 



incomplete just referred to, and the lining may be smooth or 

 cellular (Clupeidae, etc.), but sometimes it is much broken up 

 and has spongy, lung-like structure (Heterotis, Gymnarchus, 

 and other forms). 



In the Ostariophysi the anterior end of the air-bladder is 

 connected by a chain of small bones, which are probably de- 

 tached portions of some of the anterior vertebrae and are 

 called the Weberian ossicles,* with the wall of a chamber in 

 the skull wall enclosing a diverticulum of the membranous 

 labyrinth ; and in other cases anterior prolongations of the air- 

 bladder reach the skull and come into immediate contact with 

 the wall of the space in which the membranous labyrinth is 

 contained. 



In the simplest cases (Myripristis, Holocentrum, Sparus, Sargus, etc.) 

 the two anterior horns of the air-bladder apply themselves to membranous 

 spaces in the bony wall of the occipital region containing the membranous 

 labyrinth. In many Clupeidae the slender anterior end of the air-bladder 

 enters a canal in the basi-occipital and divides into two narrow branches. 

 Each of these dilates within the bone and divides again into two, each of 

 which forms a spherical swelling. A process from the vestibule (utricle) 

 of the membranous labyrinth comes into contact with these vesicles ; 

 moreover, the vestibules of the two sides are connected by a transverse 

 canal. In the Ostariophysi, in which the connection is effected by a chain 

 of ossicles, a few of the anterior vertebrae are ankylosed together and 

 modified in certain of their parts, some of which are partially detached 

 to form the auditory or Weberian ossicles. These are four in number, 

 of which three, the tripus (malleus, Fig. 119, 11), intercalarium (incus, 9) 

 and scaphium (stapes 8) form a chain connecting the air-bladder with 

 the membranous labyrinth. In addition there is a fourth the claus- 

 trum partly dorsal to and partly in front of the scaphium, which, how- 

 ever does not form part of the chain, but simply lies in the wall of the 

 atrium (see below). 



The first vertebra is represented by the centrum only which is distinct 

 from but firmly connected to the skull and the next centrum. The second 

 centrum (10) although it shows no marked sign of being composite, con- 

 sists of the completely fused centra of vertebrae -2, 3 and 4-. It is 

 therefore called the complex centrum. To the hind end of the complex 

 centrum the three next centra may be united, but these remain distinguish- 

 able. The saccule (15) of the membranous labyrinth gives off a process, 

 the ductus endolymphaticus (4), which unites with its fellow in the middle 

 line, and gives off posteriorly from the point of union a median sac, the 

 saccus endolymphaticus (5). The saccule and saccus lie in excavations 

 of the basioccipital bone, called respectively the foveae sacculi (10) and 

 the cavum sinus imparis (14). These are partly separated from one 



* E. H. Weber, De aureetauditu hominis etanimalium,Fars I.,De aure 

 animalium aquatilum, Leipzig, 1820. T. W. Bridge and A. C. Haddon, 

 The air-bladder and Weberian ossicles in the Siluroid Fishes, Phil. Trans., 

 184, 1893, p. 65-333. 



